{"id":12056,"date":"2014-03-08T21:46:30","date_gmt":"2014-03-08T21:46:30","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=12056"},"modified":"2014-03-10T06:36:58","modified_gmt":"2014-03-10T06:36:58","slug":"the-mechanism-of-diazo-coupling-more-hidden-mechanistic-intermediates","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=12056","title":{"rendered":"The mechanism of diazo coupling: more hidden mechanistic intermediates."},"content":{"rendered":"
\n

The diazo-coupling reaction dates back to the 1850s (and a close association with Imperial College via<\/em> the first professor of chemistry there, August von Hofmann<\/a>) and its mechanism was much studied in the heyday of physical organic chemistry.[1]<\/a><\/span> Nick Greeves, purveyor of the excellent ChemTube3D<\/a> site, contacted me about the transition state (I have commented previously<\/a> on this aspect of aromatic electrophilic substitution). ChemTube3D recruits undergraduates to add new entries; Blue Jenkins is one such adding a section on dyes.<\/p>\n

\"diazonium\"<\/a><\/p>\n

The mechanism can be rate limiting either in the initial electrophilic attack (black arrows) or in the subsequent proton removal (red arrows using an intermolecular base such as chloride anion).[2]<\/a><\/span>.\u2021<\/sup> The product is normally assumed to be the trans<\/em>-diazo compound rather than cis<\/em>. This distribution is certainly true in the crystal structure database (below, although some examples of cis<\/em> are known, including azobenzene itself). Would this distribution be reflected in the transition states? Initial attempts by the ChemTube3D team had resulted only in a cis<\/em>-transition state being located, and they asked me to check this out.<\/p>\n

\"diazo\"<\/a><\/p>\n

\u03c9B97XD\/6-311G(d,p)\/SCRF=water calculations using phenyl diazonium chloride (I do like my counter-ions) coupling to benzene resulted in location of both cis<\/em>[3]<\/a><\/span> and trans<\/em>[4]<\/a><\/span> transition states, the former being the lower<\/strong> by 1.0 kcal\/mol in free energy (this might well be due to the dispersion stabilisation from \u03c0-\u03c0 stacking).\u2020<\/sup> The IRC for the cis<\/em> is shown below.[5]<\/a><\/span><\/p>\n

\"cis-diazo\"<\/a>\"cis-diazoE\"<\/a>\"cis-diazoG\"<\/a><\/p>\n

You can see that the entire process is concerted. The Wheland intermediate normally invoked as part of the mechanism of aromatic electrophilic substitution is not a proper intermediate but a hidden one for the reaction with X=Y=H. The reaction coordinate has a flat top, and that passage along this part represents the hidden Wheland. The reaction barrier is high however, and it is certainly observed that only activated arenes (phenols, anilines, X,Y=OH, NH2<\/sub>) actually couple with diazonium cations. For these, the hidden intermediate is stabilized by the substituent, and no doubt emerges as a real intermediate.<\/p>\n

For my thesis work, I studied[2]<\/a><\/span> diazo-coupling of indoles<\/a>. I might have a go at returning to that work, to see if calculations can replicate my finding, that for unhindered indoles proton removal from the Wheland intermediate is fast, but add a few t-butyl hindering groups and it becomes slow.<\/p>\n

\n

PS<\/b>. Here is the IRC for the formation of trans<\/em>-diazobenzene.[6]<\/a><\/span><\/p>\n

\"trans\"<\/a><\/p>\n

\n

\u2021<\/sup>Such diazo compounds make up a significant proportion of the 50 or so real molecules I have personally added to the collection of 84 million or so thus far identified.<\/p>\n

\u2020<\/sup>Working with ions has one statistical problem that covalent systems do not have; where to geometrically place the counter-ion. One should really stochastically explore reasonable locations before concluding the likely location of the globally lowest energy pose.<\/p>\n

References<\/h2>\n
  1. S.B. Hanna, C. Jermini, H. Loewenschuss, and H. Zollinger, \"Indices of transition state symmetry in proton-transfer reactions. Kinetic isotope effects and Bronested's .beta. in base-catalyzed diazo-coupling reactions\", Journal of the American Chemical Society<\/i>, vol. 96, pp. 7222-7228, 1974. https:\/\/doi.org\/10.1021\/ja00830a009<\/a>\n\n<\/li>\n
  2. B.C. Challis, and H.S. Rzepa, \"The mechanism of diazo-coupling to indoles and the effect of steric hindrance on the rate-limiting step\", Journal of the Chemical Society, Perkin Transactions 2<\/i>, pp. 1209, 1975. https:\/\/doi.org\/10.1039\/p29750001209<\/a>\n\n<\/li>\n
  3. H.S. Rzepa, \"Gaussian Job Archive for C12H11ClN2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.956138<\/a>\n\n<\/li>\n
  4. H.S. Rzepa, \"Gaussian Job Archive for C12H11ClN2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.956139<\/a>\n\n<\/li>\n
  5. H.S. Rzepa, \"Gaussian Job Archive for C12H11ClN2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.956209<\/a>\n\n<\/li>\n
  6. H.S. Rzepa, \"Gaussian Job Archive for C12H11ClN2\", 2014. https:\/\/doi.org\/10.6084\/m9.figshare.956213<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The diazo-coupling reaction dates back to the 1850s (and a close association with Imperial College via the first professor of chemistry there, August von Hofmann) and its mechanism was much studied in the heyday of physical organic chemistry. 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